Many electronic devices, including cell phones and tablet computers, include a touch screen as an input device. A touch screen can include an array of sensors disposed over a display for detecting the proximity of objects (i.e., touches on the surface of the touch screen).
For some devices, in particular large sensor arrays, a desirable feature can be that of “large” object (LO) detection. LO detection can identify the proximity of objects over a certain size, and prevent such objects from triggering input events. For example, it can desirable to be able to rest ones palms or a drinking container (i.e., mug) on a surface of a touch screen without triggering an input event, while at the same time, still being able to control the device with finger touches.
FIG. 15A is a diagram showing conventional LO detection in a capacitance sensing system 1500. Conventional system 1500 can include an array of sensors, each of which can generate a count value corresponding to its capacitance. FIG. 15 shows count values for each position in a sensor array. Conventional LO detection can find a local maximum count value (126) 1599, and then find a contiguous region surrounding the local maximum that falls within a LO detection threshold. In the example shown, a LO detection threshold can be 50% of the local maximum, yielding a LO threshold region 1597 in the sensor array. If region 1597 exceeds an LO size minimum, the region can be considered an LO region. Touches within the LO region can then be ignored.
Conventional LO detection can provide adequate large object detection for flat, stationary objects. However, for large objects that move, have irregular surfaces, or change shape, conventional large object detection can generate erroneous inputs events.
Conventionally, detection of large objects having an irregular surface may be improved by lowering a threshold value. However, as shown in FIG. 15B, raising a threshold value can result in loss of sensitivity to small objects.
FIG. 15B is a diagram showing how lowering a threshold can result in false LO detection. In FIG. 15B, four small objects (e.g., fingers) can be in proximity to the sensor array at locations 1595-0 to 1595-3. An LO detection threshold has been lowered to 25% of the local maximum at 1595-0 (122), to detect irregular large objects. Consequently, the proximity of four separate “small” objects (1595-0 to 1595-3) can be misinterpreted as a large object region 1597′.
Conversely, the raising of a large object detection threshold can result the failure to detect a large object. This is shown in FIG. 15C.
FIG. 15C is a diagram showing how raising a threshold can result in the failure to detect a large object. In FIG. 15C, a large object can be in proximity to the sensor array in a “true” large object region 1597 (shown by a dashed line). However, an LO detection threshold has been raised to 75% of the local maximum at 1599 (74). As a result, an LO detection operation detects region 1593 (shown by a solid line), which can be smaller than an LO size threshold. Consequently, the proximity of the large object can erroneously sensed as valid input events.
As noted above, large objects that move can also present challenges to conventional LO sensing systems. A moving large object can move beyond its originally sensed region, and be misinterpreted as a touch event at the periphery of the previous large object location. This can be particularly, true if the large object moves in the same or opposite direction of a scanning of the sensors. Still further, other large objects (e.g., palms) may change shape slightly, or later pressure. In conventional LO sensing systems, such changes can also trigger false input events (e.g., false touches).